The present invention generally relates to video memory devices, and more particularly to a video memory device comprising semiconductors for storing or memorizing one field or one frame of video signal. For example, a delay device is a device for temporarily storing a signal and reading out and producing the stored signal after a predetermined time period has elapsed, and a solid-state image sensing device is a device in which a signal subjected to photoelectric conversion is temporarily stored and the stored signal is successively read out. Accordingly, the device referred to as the video memory device in the present specification also includes such devices.
Generally, video information in terms of frame periods of a video signal is extremely similar, and the correlation among frames is quite high. On the other hand, there is almost no correlation among frames with respect to noise included within the video signal. Hence, if the video signal is averaged in terms of frame periods, the energy of the signal component of the video information hardly changes while only the energy of the noise component decreases, and noise suppression is carried out as a result. A noise suppression circuit for carrying out such a noise suppression, is designed to carry out subtraction between the video signal and a signal obtained by delaying the video signal by one frame, and then carry out subtraction between the subtracted result and the video signal. Thus, in this type of a noise suppression circuit, it is necessary to use a one-frame delay device.
The conventional noise suppression circuit was designed as a recursive filter comprising a one-frame delay circuit. In this conventional circuit, it was necessary to provide an analog-to-digital (A/D) converter for converting the input video signal which is an analog signal into a digital signal and a digital-to-analog (D/A) converter for converting an output digital signal into an analog signal. Further, the one-frame delay circuit was constituted by a frame memory for storing one frame of the digital signal obtained from the A/D converter.
For example, when converting a video signal having a field frequency 60 Hz by the A/D converter with a sampling frequency 4f.sub.sc (where f.sub.sc is a chrominance subcarrier frequency and equal to 3.579545 MHz, for example) into a digital signal with a quantization number of eight bits, and storing this converted signal into the frame memory, the number of bits required in the frame memory becomes an excessively large number as follows. EQU 4.times.3.579545.times.8.times.1/30 =3.818 Mbits
Hence, if this frame memory is constituted from a 64-Kbit random access memory (RAM), sixty of such RAMs become necessary. Accordingly, the conventional delay circuit required a large number of RAMs, and the cost of the circuit became high and the size of the circuit became large. In addition, because the costs of the A/D converter and the D/A converter are high, there was a disadvantage in that the cost of the noise suppression circuit became high and the size of the noise suppression circuit became large.
On the other hand, 1H-delay elements are known and reduced to practice, for delaying the video signal by one horizontal scanning period (1H) by use of charge-coupled devices (CCDs). Hence, it is possible to design a delay device for delaying the video signal by one frame, by connecting 525 of such 1H-delay elements in series. However, the 1H-delay element comprises N (N is an integer) stages of CCDs, and when signal charges are transferred serially through the 525 1H-delay elements, the transfer number becomes equal to 525N which is an exceedingly large number. Morover, because the transfer efficiency is not 100%, the signal becomes attenuated during this exceedingly large number of transfer. As a result, the signal-to-noise (S/N) ratio of the signal finally obtained after the delay of one frame is extremely degraded, and cannot be used for practical purposes. Therefore, the delay device having such a design could not be used for practical purposes.
Recently, accompanied by the rapid development in the field of semiconductor technology, there has been an active development in solid-state image sensing devices which do not use electron beams. Compared to the image sensing device employing the image pickup tube, the solid-state image sensing device is smaller in size and lighter in weight, and the power consumption is small because the required voltage is low. Further, the solid-state image sensing device is advantageous in that the solid-state image sensing device is strong in resisting mechanical shocks or vibrations, the serviceable life of the device is long providing high reliability, and there is no need to subject the device to baking. However, during the manufacturing stage of the solid-state image sensing device, inferior image sensor parts are easily produced, and the yield rate becomes a serious problem. Hence, there was a disadvantage in that the manufacturing cost of one solid-state image sensing device became high.
Conventionally, when the image sensor part is inferior, such an inferior image sensing device could not be used as the image sensing device and was destroyed as an inferior product. However, if such inferior products may be used for other purposes so that such inferior products are effectively used, the manufacturing cost of the solid-state image sensing device may be reduced as a result.
On the other hand, image sensor cameras built-in with recording and reproducing apparatuses are being developed for practical use, and in such cameras, there is a demand for compact size, simple construction, and low cost.